The optimization of growth conditions for high quality 1.3 &mgr;m InAsP/InGaAsP laser structures by gas source molecular beam epitaxy is reported. Measurements of photoluminescence intensity and threshold currents of highly strained InAsP/InGaAsP multiple quantum well (MQW) lasers indicate an optimum growth temperature substantially lower than that for conventional 1.3 &mgr;m InGaAsP quaternary lasers. Broad‐area laser structures grown under the optimum conditions exhibited threshold current densities as low as 400 A/cm2. Buried heterostructures with uncoated facets exhibited threshold currents as low as 5.0 mA and internal losses of 8.0 cm−1. ©1995 American Institute of Physics.